Coastal sources, sinks and strong organic complexation of dissolved cobalt within the US North Atlantic GEOTRACES transect GA03

Abstract

Abstract. Cobalt is the scarcest of metallic micronutrients and displays a complex biogeochemical cycle. This study examines the distribution, chemical speciation, and biogeochemistry of dissolved cobalt during the US North Atlantic GEOTRACES transect expeditions (GA03/3_e), which took place in the fall of 2010 and 2011. Two major subsurface sources of cobalt to the North Atlantic were identified. The more prominent of the two was a large plume of cobalt emanating from the African coast off the eastern tropical North Atlantic coincident with the oxygen minimum zone (OMZ) likely due to reductive dissolution, biouptake and remineralization, and aeolian dust deposition. The occurrence of this plume in an OMZ with oxygen above suboxic levels implies a high threshold for persistence of dissolved cobalt plumes. The other major subsurface source came from Upper Labrador Seawater, which may carry high cobalt concentrations due to the interaction of this water mass with resuspended sediment at the western margin or from transport further upstream. Minor sources of cobalt came from dust, coastal surface waters and hydrothermal systems along the Mid-Atlantic Ridge. The full depth section of cobalt chemical speciation revealed near-complete complexation in surface waters, even within regions of high dust deposition. However, labile cobalt observed below the euphotic zone demonstrated that strong cobalt-binding ligands were not present in excess of the total cobalt concentration there, implying that mesopelagic labile cobalt was sourced from the remineralization of sinking organic matter. In the upper water column, correlations were observed between total cobalt and phosphate, and between labile cobalt and phosphate, demonstrating a strong biological influence on cobalt cycling. Along the western margin off the North American coast, this correlation with phosphate was no longer observed and instead a relationship between cobalt and salinity was observed, reflecting the importance of coastal input processes on cobalt distributions. In deep waters, both total and labile cobalt concentrations were lower than in intermediate depth waters, demonstrating that scavenging may remove labile cobalt from the water column. Total and labile cobalt distributions were also compared to a previously published South Atlantic GEOTRACES-compliant zonal transect (CoFeMUG, GAc01) to discern regional biogeochemical differences. Together, these Atlantic sectional studies highlight the dynamic ecological stoichiometry of total and labile cobalt. As increasing anthropogenic use and subsequent release of cobalt poses the potential to overpower natural cobalt signals in the oceans, it is more important than ever to establish a baseline understanding of cobalt distributions in the ocean.

The acceleration of dissolved cobalt's ecological stoichiometry due to biological uptake, remineralization, and scavenging in the Atlantic Ocean

“…The aggregate slope of this correlation is often described as "ecological stoichiometry" for its inferred biological usage across a diversity of organisms present (Sterner and Elser, 2002). An emerging feature and mystery regarding cobalt relative to other macro-(N and P) and micronutrients (Zn and Cd) is its unusually large range in stoichiometries, spanning more than an order of magnitude from 29 µmol : mol in the central North Pacific to 560 µmol : mol in the equatorial Atlantic (Noble et al, 2012Saito et al, 2010;. That high equatorial Atlantic value has long appeared to be an outlier, and these putative high stoichiometries are a focus of this study.…”

“…Some of the available datasets include several process studies of Co and Mn radiotracer uptake into biotic particles in the Sargasso Sea and coastal environments (Lee and Fisher, 1993;Moffett and Ho, 1996), a study of Co and Mn precipitation in anoxic fjords (Tebo et al, 1984), and laboratory experiments demonstrating that manganeseoxidizing bacteria also oxidize Co under aerobic conditions (Lee and Tebo, 1994). These few studies, combined with early observations of the depletion of dissolved cobalt in intermediate and deep waters (Knauer et al, 1982;Martin et al, 1989), and observations of a "cobalt curl" in the plots of dissolved cobalt and phosphate space showing preferential removal of Co vs. P (Noble et al, 2012;Saito et al, 2010), have contributed to this notion that scavenging is an important process. A recent study put forth a contrary argument that scavenging is less important in the cycling of dissolved cobalt, and that instead cobalt depletion in the ocean interior can be better explained by physical and remineralization processes (Dulaquais et al, 2014b).…”

“…More recently, studies have identified deviations from Redfield's elemental ratio (Martiny et al, 2013), as well as extended the biological stoichiometry to metal micronutrients in some microorganisms (Ho et al, 2003;Outten and O'Halloran, 2001;Sunda and Huntsman, 1995). Cobalt has the distinction of being both the scarcest of metal micronutrients in the oceans and of having perhaps the most variable of elemental stoichiometries in both dissolved and particulate phases (Saito et al, 2010;Noble et al, 2017). This paper aims to characterize cobalt's unusual behavior, both for the purpose of improving the knowledge of cobalt biogeochemistry and to further our general understanding of the processes that drive the connection between elemental environmental distributions and cellular biochemistries.…”

“…Improved analytical methods for the determination of dissolved Co, and the resulting recent production of large GEO-TRACES datasets (Geotraces Group, 2015), provide new opportunities to explore the variability in Co stoichiometry and the processes that create this range (Baars and Croot, 2015;Bown et al, 2011Bown et al, , 2012Dulaquais et al, 2014a, b;Hawco et al, 2016;Noble et al, 2012Noble et al, , 2008Saito et al, 2010;Shelley et al, 2012). Here we examine and compare two zonal full depth sections of total dissolved cobalt and labile cobalt from the North and South Atlantic Ocean to examine the tug-ofwar among competing processes affecting cobalt cycling in the oceanic water column.…”

“…Specifically, we have developed and employed a fine-scale Redfieldian statistical analysis of the variation in the stoichiometry of cobalt relative to phosphorus across vertical and horizontal dimensions to discern biogeochemical processes influencing dissolved cobalt in the Atlantic Ocean. This paper is a companion to that of Noble et al (2017), describing the sources and distributions of dissolved cobalt and its chemical speciation in the US North Atlantic GEOTRACES GA03 section.…”